Pressure measuring apparatus



June 16, 1942 R. 1'. HURLEY 2,286,621

PRESSURE MEAsURmG APPARATUS Filed Dec. 23, 1958 2 Sheng-,sheet 1 lvm @amam Patented June 16, 1942 UNITED STATES PATENT OFFICE PRESSUREMEAsUnINGAPrAnATUs Roy T. HurleyQDobbs Ferry, N. Y., assignorl to BendixAviation Corporation, South Bend, Ind., A a corporation of Delaware lApplication December A23, 1938, Serial No. 217,379

15 Claims.

This invention relates to measuring devices and more particularly to apressure measuring device utilizing the light interference principle foreffecting accurate determinations of barometric pressures and altitudes.1

An object of the-present invention is to provide Ia novel measuringdevice employing interference fringes to determine the differencebetween the pressure of a given-medium and that of a fixed standard suchas a vacuum.

means embodied therewith for readily checking and reproducing thestandard against which measurements are made and for Calibrating thelindicating mechanism.

A further object ,is to provide a novel device operable as an altimeterto give an altitude reading relative to a standard predeterminedaltitude.-

A still further object of the invention is to provide a novel apparatusfor dividing and recom- (cl. :is-14) 'taken substantially along line 5-5of Fig. 2v showing a mechanism adapted to vary the optical bning a lightbeam wherein the divided rays of the beam travel over paths havingoptical lengths which remain substantially unaffected by ordinarychanges in temperature.

Still another object is to provide a novel pressure and altitudemeasuring device adaptable for use as a eld instrument to give accuratereadings under conditions of varying temperatures.

' The above and other objects and novel features of this invention will`more fully appear from the following detailed description when the sameis read in connection with the accompanying drawings. It is to beexpresslyunderstood, however, that the drawings are for purposes ofillustration only and are not intended -as a definition of the limits ofthe invention, reference being primarily had for this latter purpose tothe appended claims.

In the drawings, wherein like reference char- 4Q' acters refer to likeparts throughoutviews:

1 is atop .plan view, with parts broken away, of one embodiment of thepresentinvention with a cover therefor removed;

VFig. 2 is a sectional-view, with parts broken away, ta-kensubstantially along line. 22 of Fig. A1;

the several Fig. 3 is an end elevation of the above ernbodiment; A

Fig. 4 is a somewhat enlarged sectional view taken, substantially alongline 4 4 of'Fig. 3 showing a form of control mechanism utilized in thepresent invention;

- `-10 Another object of the invention is to provide af compactmeasuring apparatus having 4novel length of an interference beam; and,Ay

Fig. 6 is a diagrammatic view showing types of electrical and opticalsystems utilized in the invention.

It is wen known thatif a beam of white light .is divided into two rays,each of which'is directed over a path of the same optical length, i. e.a length which is a direct function of the index of refraction of. themedium of travelfas well as of the geometric length thereof, and theraysare thereafter combined into a singlebeam-interference fringes areproduced which have a'single black fringe centrally located relative tothe other colored fringes. If the optical length ofl one of the paths isthereafter varied, for example by a variation in the index of one of themediums of that path, the black fringe appears displaced from itscentral position. To compensate for this variation and bring Vthe blackfringe back toits central position, the geometric. length of one of themediums of the changed path may be varied. A measure of this change ingeometric length is a measure ofthe index variation producing theoriginal change in the optical length. Thus, if a pressure change causedthe variation in index for whichcompensation is made, this -pressurechange may be measured by measuring a linear distance, namely theincrease or decrease in geometric length of the compensating medium.Accordingly, in the present invention novel means are provided fordividing a beam of light into two rays and for causing one of said raysto travel a path comprised'inpart of a medium of predetermined opticalIeng'th such as a vacuum and causing the other ray to travel a pathcomprised in part of'a medium such as atmospheric air whose pressure is'to be measured, said lastnamed path having an unknown optical length'buta Vknownfgeometric length." Means are provided in the path containingvthe medium 0f unknown pressure whereby compensation/for the differencein optical lengths of the two/paths due ,f tothe Adifference in pressureof their mediumsy may be effected. The length ofcompensating mediumwhich must be introducedto equalize the optical paths is a measureof-the pressure o difference between the above mediums. This differencewill' be indicated as'an absolute pressure of the unknown medium whenthe standard medium against which comparison is made is a vacuum. v

In thesingle embodiment of the present inven- Fig. 5 is 'a somewhatenlarged` sectional view 55 tion illustrated, by Way 0f example. aportion of -with supports'IZ of yielding material xed to thebase-thereof. To-maintain a uniform temperature within said casing underall external conditions, the latter, together with cover I I, isprovided with a lining I3 of suitable insulating material such as therubber compound commercially known as Vacoboard, said liningbeingpreferably cemented to the interior of the walls of said casing and saidcover.

A suitable massive base `or frame I4 supported at three points by legsI5 is mounted within casing I0, said legs beingsecured to said casing bymeans of suitable vibration eliminating members I6. Each of the lattercomprises an annular socket I'I (Fig.`2) preferably formed from a heatinsulating material such as Bakelite and secured to said casing byscrews I8. A disc orv plate I9 of yielding material, such as rubber, issupported within said socket by an annular flange 2U which is fixed tosaid disc and is mounted in a groove 2I in the wall of said socket.Bolts 22 extending through said discs are threadedly secured vto legsI5. Vibrations and sudden shocks imparted to casing I are thus absorbedby the above structure and frame I4 is unaffected thereby.

In order to provide optical paths having equal geometric lengths for themediums of unknown and known pressures, a bar 23 of accurately plates.The unit comprising said plates and said separators is mounted upon abracket 43 secured by screws 44 to supporting frame I 4. a

A portion-45 of the surfacel of plate 40 adjacentA plate 4I is coatedwith a thin film of metal,

incandescent bulb 41 (Fig. 6) passing into casing knownlength havingparallel openings 24 and 25 extending the length thereof is mounted uponframe I4, being secur'ed thereto by screws 26. A pair of transparentglass plates 2'I and 28 are wrung or cementedto the ends of bar 23creating chambers of equal geometric length within said bar which areisolated from the interior of cas- Cil I 0 through a window 48 (Fig. 2)in the wall of said casing, strikes lm 45, at an angle of approximately45 and is divided into two rays BC and BD.

Ray BC, which is reflected from said film,

`strikes surface 46 and is totallyireflected through the unsilvered'partof plate 40 and plate 2'I into chamber 24. Ray BD passes through plate40 and plate'2'I into chamber 25, traversing Vthe length of said chamberand passing therefrom to a polished surface-49 of a metal block 50 whichis secured to frame I4and from which said ray is reflected back alongits path to the plane of separation designated by point B. Ray BC issimilarly caused to retrace its path back to the plane of separationafter traversing the length of chamber 24, being reflected by a lm I ofmetal on the exterior surface of the portion of plate 28 which coverssaid last-named chamber. The distance traveled by ray BD after passingthrough plate 28 is substantially equal to the additional distancetraveled by ray BC (distance BC in Fig.

'- `1) between 1:lates'4ll and 4I, so that the rays ing I0. Plates 21Vand 28 have parallel surfaces so that equal thicknesses of of chambers24 and 25. Y A conduit 29 (Figs. 2 and 6) preferably flexible glass formthe ends and having a low coefficient of thermal conduc` a manually op-30, the latter being mounted on a.-

tivity connects 'chamber 24 with erable valve panel 3I secured to thefront end of Vcasing IIJ. A conduit 32 connects said valve, which isnormallyclosed, to a suitable means such as a vacuum pump 33\(Fi g. 6)for evacuating chamberv 24. AA conduit 34 (Fig.- 6) similar to conduit29 connects chamber toa -valve 35V (Fig. 3) mounted onpanel 3| and saidvalve, which is .normally open, is connected by a second conduit 36(Fig. 6) to a point at which a pressure measurement is to be made. Todry the small quantities of air owing into said last-named cha`m`= berwith pressure increases, an absorption cham- 'ber or bulb 31 (Fig. 6) isinterposed in conduit 36.

. A pair of Geissler tubes 38 and 39 (Fig 3) communicating directly withconduits 29 and 34, re-

spectively, are-supported by valves and 35 and when a high potential isapplied across the terminals of one of said tubes the luminous dischargeproduced therein is an accurate measure of .the degree of vacuumexisting in the chamber to which said tube is connected.

Means are provided for dividing a beam of light into two rays ofsubstantially equal intensities and for directing said rays intochambers 24 and 25, said-means comprising a pair of transparent plates40 and 4I (Figs.y land 6) preferably of quartz having accurately planedparallel surfaces and fixed-in parallel relation to each other at apredetermined distance from each other by a pair of parallelsurfacedquartz separating pass over paths of equal geometric length and,except-for the mediums within chambers 24 and 25, through substantiallyequal lengths of like mediums.

The rays are reunited at B and' by means of their interference orreenforcement produce the fringes whereby the optical lengths of thepaths of said rays may be compared. Beam BE, formed by the reunitedrays, passes upward (as viewed in Figs. 1 and 6) through plate 40 andpreferably through an achromatic lens 52 into a hollow tube or cylinder53 suitably mountedon brackets 54 secured to frame I4. A prism 55mounted within said tube. in the path of beam BE reflects the latteralong the length of-said tube to a second prism 56 (Fig. 6) mountedtherein, and said last-named prism reflects the beam in turn through anachromatic lens 5'I to a telescope eyepiece 58 through which anobserver` may view the interference fringes.

Telescope 58 is surface -49 of block 50 and surface 5I of plate 28,and'a line is engraved or a cross hair is stretched across one of saidsurfaces in such a manner that when it is viewed through said telescope.it is parallel to the fringes and thus prodischarge ofhelium gas.

condenser lens 59 (Figs. 3 and 6) adapted to plates 42 interposedbetween'and wrung to` said 7 5 vides a'reference line by which the blackfringe may be centered.' Due to this arrangement, ac-

cidental movement of telescope 58.dces not vitiate vthe relativepositions of the fringes and the reference line.

It is desirable at times, for instance when cali-y brating theindicating scales, to have the interference fringes produced by thereenforcement of monochromaticl light of a known wave length such as thelight emanating from the 'luminous gather and concentrate rays of lightinto a beam vnormally focused on' polished For this purpose, a

yyond the surface thereof.

f tube 60 iilled with a suitable gas, such as helium,

is mounted on said panel and has the central cylindrical portion.thereof extending across said lens and when a high potential is appliedacross the terminals of said tube a luminous discharge of monochromaticlight is produced.

A beam of this light passes through lens 58 to-entr casing I and passesinto tube 53 through a mask 6| (Figs. 1 and 6) secured to the end ofsaid tube, said mask having a slot 62 therein which directs said beambeneath prism 56 and onto prism 55. From prism 55 the beam'is-reiiected(Fig. 6) substantially along path EB to the planeof separation, i. e.the partly coated portion 45 of plate 40, where said beam is divided andthe rays thereof directed through chambers 24 and 25. Because of theslight angle at which said beam is directed through tube 53, part of therecombined beam is reected from prism 55 to prism 56 instead of back tomask 6|. 'I'hus the fringes which are of one colorand uniformly spacedas distinguished from the vari-colored white light fringes may be viewedthrough eyepiece 58. a I

Novel means are provided to vary the optical length of vone of the pathsof the divided rays BC and BD and thereby cause said paths to bel ofequal optical lengths despite the difference in the pressures of .themediums inchambersv24 and 25., y means are applied to path BD and asshown comprise a transparent plate 63 (Fig. 6) of iluorite interposeddnsaid path between plate 28 and polished surface 48 of block 50. A frame64.(Fig. carries plate 63, the latter being preferably cemented thereinand said frame is ,pivotally mounted between conical pivots 65 and 66.The latter are freely mounted in openings' provided therefor in abracket or. support 61 which is secured to-frame I4 by screws 68, lowerpivot 65 being held in engagement by a collar 68 integral therewithwhich engages the surface.

vof said bracket and upper pivot 66 being urged into engagement withframe 84 by a cantilever spring 10. The latter is' fixed at one end tosaid bracket by means such as `screws 1|, and has i the free end thereofengaging and applying a pressure to the upper end of pivot 66 which ex-In the illustrated embodiment, said radially from the axis of rotationof said frame and has a downwardly extending portin 16 which carries abutton or contact k11 formed block maintains the engagement between saidv contact and said rod.

Red 1s (Figs. 1 and 2) is the .cured to supporting frame |4 by a bracket82 and a rotary barrelv 83 into which said drum telescopes and whichmotivates rod 18. A scale 84 is calibrated o n the leading edge ofbarrel 83 and cooperates with a scale 85 axially calibrated on thesurface of drum 8| to give an extremely accurate reading of the amountof motion imparted to rod 18 and consequently of the amount of iiuoriteintroduced into optical path BD,

In order that scales 84 and 85 may be accurately read, a magnifying lenssystem 86 (Fig. `2) through which said scales are viewed, is provided incover and comprises lenses 81 and 88 carried by a tubular member 88composed of a heat insulating material such as Bakelite, said memberbeing fixed to said cover by screws 80.

from the exterior of casing l0, a readily accessible knob 8| is mountedupon a shaft 82 which extends through a housing 83 secured topanel 3|into the interior of-'said'casi'ng To protect micrometer 80 from thetransmission thereto of shocks and vibrations which are applied to knob8| and to insure vagainst misalignment of said micrometer, shaft 82 isconnected to barrel 83 tends through said bracket a short distanceA be-When frame 64 is pivoted, the length of fluorite in the path of ray BDis varied, varying the optical length of that path.

To cause the change in'optical length of path BD to be due to adifference. in the lengths of iiuorite inthe two paths, a plate offiuorite 12 (Figs. 1 and 6) is provided in path BC between plate 40 andplate 21, Asaid iluorite plate being carried by a bracket 13 attached tobar 23 lby means of screws 14. By interposng fluorite in both paths itremains possible to have said paths of equal optical length when themediums in the chambers are alike and, furthermore, the presence ofiluorite in both paths of the optical system produces interferencefringes which are very distinct and clear.

In order to control the pivotal movements of frame 64 and simultaneouslymeasure the amount of medium thereby introduced in the optical path,novel .means operable from the exterior of casingI |0 are provided.An'arrn 15, (Figs. l

and 5) secured to the face of frame 64 extends by a collapsible member,such as --a Sylphon 84,

. adapted to transmit a torque but notan axial load, one end of saidSylphon being xed to said shaft and the other end being secured to saidbarrel.

Novel means are provided for limiting the rotary motion of shaft 82,therebypreventing an overstressing of micrometer mechanism by aninadvertent attempt to move the same n beyond thev allowable limits ofmotion therefor, said means comprising a worm 85 (Fig. 4) formed withshaft 82 and a gear 86 mounted within housing 83 and operativelyengagingvsaid worm. A pin 81 is fixed to or formed integrally'with gear86 on the face thereof, being movable therewith in a circular path, anda stationary vpin or lug 98 secured to housing 83extends `downwardlytherefrom into the path of motion of saidI firstnamed pin, thus limitingthe rotation of said gear in either direction to slightly less than onerevolution. The translatory motion of rod 18 of micrometer mechanism 80is correspondingly limited.

5` To determine the temperature within casing I0, a pair of thermometers88 and |00 (Figs. 1

and 2) are provided and calibrated stems I0| and |02 thereof are securedto a strip |03 of insulating material, such as Bakelite, by clamps or 0brackets |04, said strip being supported upon bar 23 by` being screwedto lugs |05 interposed between said bar and said strip. Stems |0| and'|02 curve downwardly at right angles to the plane of,strlp |03 and passthrough openings |06 .reciprocatingelement of an accurate micrometermechanism- 80. comprising a stationary drum 8| rigidly se- For thepurpose of actuating micrometer 80 tending portions of said stemsi:oinbulbs |01 and |08 which are' parallel to the plane of said stripsaid bulbs being encasedin. a block |09 secured to bar 23,. An accuratetemperature reading may thus be obtained from said thermometers sincethe latter are calibrated over successive temperatureranges; forexample; the' scale of thermometer 99 reads from 0f to 25 C. and that oftherf mometer from 20 to 35 C., thusproviding an` over-all range of from0 to 35 C.

-a chart, the latter method being preferred where the instrument is tobe employed under conditions of varying temperatures.

In-operation, chamber 24 is evacuated to a standard vacuum by pump 33and valve 30 is closed, and thereafter chamber-25 is opened toatmospheric air by the opening of valve 35, the

A pair of transparent glass .plates 0 andi I I A screws ||5. A strip 6of yielding insulating. material, such as rubber, is interposed alongthe edge of opening ||`2 between plate vand strip |03, preventing theconduction of heat into the interior of casing |0 through metal brackets||4.

pressure within said last-named chamber thus becoming equal to that of'the atmosphere. White lightfrom bulb 41 is admitted into casing l0,divided and recombined and the images produced thereby are viewedthroughtelescope 58.

The fringes of white light appear only when the optical paths of thedivided rays are substantially equal andthe black fringe of said fringesis centrally located only when the' optical paths are exactly equal.Thus, micrometer 80 is actuated through knob 9| topivot frame 64 andvary the An electrical circuit -suitable for applying a high potentialacross vthe terminals of Geissler tubes 38, 39 and 60 is shown in Fig. 6and comprises a source of electrical energy ||1 connected across the lowpotential side of a transformer 8. A lead ||9 from the high potentialside of said transformer is connected to a common terminal of conductors|2l, |22 and |23, each of said conductors being connected to a terminalv'Ihe other ,ter-

of one of said Geissler tubes.

minals of the latter are connected by conductors |24, and |28 .toseparatecontacts of a three- The pole of saidv way single pole switch|2I.v switch is connected by lead |28 to the other ter- -mnal of thehigh potential sideof transformer |I8. Thus, a high potential may be.applied across any of said Geissler tubes.

Prior to use, the instrument is readily calibrated by exhausting chamber25 to a vacuum equal tov that in the standard chamber 24, said.

vacuums being accurately checked by applying high potentials across theterminals o'f Geissler tubes as and sa and observing the mminousdischarge produced y:within said tubes. Air is' thereafter slowly,admitted into chamber 25 until the pressure within said Achamberattains 'aknown value. During the admission of Vair into said chamberlthe interference 'fringes produced by monochromatic light, i. e. theluminous discharge of Geissler tube 60, are observed through teleamounto f 'fluorite in -optical path BD until interference fringes becomeapparent through telescope 58 and the black fringe thereof is centrallylocated.- Y Y .When the optical paths are thus equalized and thetemperature has remained substantially equal to that at which theinstrument was calibrated, scales, 84 and'85 of 'micrometer mechanism 80record the pressure Vdifferential equal to the change in optical lengthfor which the uorlte Vhas compensated. However, when the instrument issubjected to changing temperatures dur.- ing use, it becomes necessaryto make a temperature correction, since the index of refractionandtherefore the optical length actually varies as a function'of thedensity and the latter varies as the pressure and temperature.Accordingly, a reading of the micrometer scales and a temperaturelreading as recorded by thermometers 99 and |00 are utilized to determinethe actual pressure from a chart or table giving the -pressure vreadings corresponding to any combination of micrometer scale andthermometer readings.

In order to make an'accurate temperature correction possible fromtemperature readings of an ordinary mercury thermometer, the temperatureeffect upon the pressure readings is decreased to a minimum by meansembodied within the instrument. The effect of the thermal expansion orcontraction upon the length ofchamber 25 is offset by having chamberr24formed in the length thereof subjected to the same thermal scope 58.Monochromatic light produces fringes of the same color and Ashaperegardless of` dilerences in optical lengths of the paths traversed bythe divided rays and said fringes move past the reference line at a ratedetermined by the rate of vchange of optical length of one of 'thedivided rays in change.

this case by the rate of pressure `By counting theifringes as-they-movepast the L reference line and knowing the geometric length of thechanging air path as well as the wave length of the -monochromaticlight, the pressure dilferenceequivalent tothe movement of apredetermined number ofA fringes Vmay be deterfmined. This movement offringesmay thereafter be reproduced by introducing uorite into the pathof one of the divided rays and thus the pressure equivalent of a certaindisplacement vof micrometer mechanism may be determined, This may bedirectly marked off on scales 84 and 85 or the reading of said 'scalescorresponding to this known pressure change may be recorded on same baras said first-named chamber and the effects. However, since the densityof a vacuum is unaffected by'temperature, there is no compensatingeffect produced within chamber 24 for 'the change indensity of themedium in chamber 25' produced by a temperature change. To compensatefor this density effect, a'. portion of the path of ray BD is caused toincrease'in length with temperature increases, while the portion of pathBC which normally equalizes said rstnamed portion is affected to alesser extent by the same temperature change. The portion of path BD soaffected is the length Vbetween surface 49 and the end'of chamber25Vwhich, as heretofore pointed out, is substantially equal to theadditionaldistance BC (Fig. V1) traveled by ray. BC ,before reachingchamberA24.' The distance between surface 49 and chamber 25 varies withthe thermal expansion of supporting frame I4 Whereas the distancebetween plates 40 and .4| and therefore distance BC depends on theseparator plates of quartz, a predetermined dilerchanges.

been successfully employed as the materials pro-K ducing thecompensating dierenoe in geometric claims.

ence in the linear lengths of said distances and therefore of paths BCand BD may be produced for diierent temperatures. 'Ihis diierence ingeometric length of the paths due to temperature changes substantiallycompensates for the di'erence in `densities of the mediums in chambers24 and 25 due to like temperature Although quartz -and 'cast iron havelengths of the paths, it will now be understood that other materialshaving suitable coefcients of thermal expansion may be used for frame I4and separatorplates 42.

-To insure against uncontrolled and unpredictable thermal expansions andeffects because of temperature differences at diierent points withincasing l0, the latter, as heretofore pointed-out, is highly insulated.Few heat conducting parts extend within said casing and consequently thetemperaturein thel latter remains uniform and is not subject to rapidchanges because of fluctuations in the temperature of the surroundingatmosphere. v

The compensation above described lis not complete, but makes possible anaccurate pressure determination by the instrument when a temperaturereading is obtained from an ordinary mercury thermometer. Thus, a tableor chart may be drawn up giving accurate pressure or altitudeequivalents for the readings directly obfor measurementv by said means,said chambers *being formed in a bar of xed length having longitudinalparallel openings therethrough and having transparent plateswrung to theparallel ends thereof.

2. In apparatus utilizing the interference of light as a basis for apressure measurement, means for varying the optical lengthof a beam oflight comprising va pivotally mounted transparent plate interposed inthe path of said beam,

mechanism adapted to pivot said plate and vary the length of said platein said path, indicating means including scales calibrated'on saidmechanism to measure the movement'imparted to said mechanism and toabsorb axial forces without transmitting the same to said mechanism. l

3. In a device tor measuring'barometric pressures, a, source of light,means for dividing a i' beam of said light from said source into twobeams, and means for varying the optical length of one of said, dividedbeams, said last-named means comprising a transparent plate interposedin the path of said last-named beam, a pivotally tained upon scales 84and 85 'and thermometers 99 and |00. Without said compensation thereading. of scales 84 and. 85 would vary so greatly with slighttemperature changes that it would be practically impossible to equip acommercialV unit with a temperature measuring device capable ofregistering the temperature tothe desired accuracy.

There is thus provided a novel barometric pressure and altitudemeasuring device wherein the density of a medium free to enter a chamberin said device is measured by the interference of a light beam, and saidmeasurement, together with a reading, of the temperature within-theinstrument, is utilized to obtain the pressure or altitude of saidmedium. lWhen said device is to be employed for laboratory purposes orotherwise used where a substantially constanttemperaturemountedsupporting frame carrying saidv plate, a Vmicrometer mechanismengaging said frame, means for actuating saidmicrometer mechanism topivot said frame and vary the length of said transparent platedn thepath of said last-named beam, and a vibration absorbing meansoperatively connecting said actuating means to said micrometermechanism. i

4. In a pressure measuring device, interferometric means for measuringthe density o f an unknown medium lincluding means for dividing a beamof light into two beams of substantially equal intensities, and, meansfor varying the length of tlie optical path of one of said beams, saidlast-named means comprising a plate of transparent material having anindexgof refraction differing from that of air, a frame membersupporting said plate in the ypath of said lastlnamed beam, means forpivotally mounting said member, 'an element secured to said member,

- means maintaining the engagement between said micrometer mechanismadapted to engage -said element to pivot said v'member and vary thelength of saidplate in the path of said beam, resilient mechanism andsaid element, actuating means,

-means for the motion of said actuating means in both directions, andvibration absorb- Although only asingle embodiment of the invention has-been illustrated and described, it is to be expressly understood thatthe same is not limited thereto. Various changes maybe made in thedesign and arrangement of` parts without departing from the spirit andscope of the invention. For a definition ofthe invention, referappendedence will be had primarily to' the What is claimed is:

l. In an instrument having interferometric means for measuring thediiference in pressure between equal geometric lengths of two mediums, apair of chambers for containing said mediums.,

ing means operatively connecting said micrometer mechanism to saidactuating means.

5. In apparatus of the class described, the comingvwith one of saidchambers, and means for applying a high potential across the terminalsof vsaid tubes to indicate the degree of vacuum in said chambers.

6. In apparatus utilizing light interference for measuring barometricpressures, a casing, a chamber of known length mounted in said casingand adapted to beevacuated to provide a known length of comparisonmedium, means for absorbing' means.

evacuating said chamber, a Geissler tube mounted on the exterior of saidcasing-and communicating with said chamber, and means for applying ahighpotential across the terminals of said tube 4to visually indicatethe degree of vacuum existing in said chamber.

7. In a measuring instrument utilizing light interference as 'a basisfor measurement, a casing, a member of knownlength supported within saidcasing and having parallelend surfaces, said member having'a .pair ofopenings therein extending the length thereof, a pair of transparentplates, one of said plates being wrung to each end of said member toform chambers of said openings, the interiors of said chambers beingisolated from the interior of said casing.' means connecting one ofvsaid chambers with the atmosphere, and evacuating means communicatingwith the other of said chambers.

8. In apparatus utilizing the light interference principle wherein abeam of light is divided into two rays, a casing, pivotal means adaptedto vary 9. In a measuring instrument,' a casing,'a supporting membermounted in said casing', a bar.

supported by said member and having a pair ofA openings extendingtherethrough, a pairof transparent closure plates for said openingssecured to the ends of said bar, said plates being parallel to eachother whereby chambers of equal length are formed in' .said bar,interferometric means for measuring the difference in density-betweenthe mediums 'contained in said chambers, said interferometric meanscomprising means for dividing a. beam of lightinto two rays, 'means forldirecting one ray through one of said chambers and the otherray throughthe other of said chambers, means for recombining said rays and meansfor directing and viewing the recombined beam, said dividing, directingand recombining means being mounted on the interior of said casing andsaidviewing means in the wall A v of said casing, means connecting oneor said chambers to the atmosphere, and means communicating with theother of said chambers for prof viding a medium y therein.

of known index of refraction 10. In apparatus of the class described,means comprising a pair of closed and substantially parallel chambershaving transparent ends, a source of light, means for dividing a beam oflight from said source into two substantially parallel rays and fordirecting one ray into one of said chambers and the other ray intothe'other of said chambers,. reflecting means disposed exteriorly of oneof said chambers for reilecting the ray passing therethrough back to thedividing and directing means, compensatingl means disposed between saidlast-named chamber and said reecting means, said .compensating meansbeing path of the ray intercepted thereby, means for reilecting theother ray back to the dividing and directing means, said dividing anddirecting other end of -said vibration means being adapted to recombinethe reflected lrays, viewing means, means for directing the recombinedbeam into said viewing means, means' for providing a medium ofknownindex of refraction in one of said chambers, means for introducinga medium of unknown index into the other chamber, means for actuatingsaid compensating means to equalize the optical lengths of the paths ofthe divided rays, and means f or indicating the amount of movementimparted to said compensating means `to accomplish -the equalization,said movement being a function of the. diierence in indices ofrefraction of said mediums. f

11. `In interferometric means for measuring the diierence in pressurebetween the mediums in a chamber containing atmospheric air and anevacuated chamber,a source of light, means for dividing a -beam of lightfrom said'source into two rays and for directing one of said raysthrough the atmosphere filled chamber, ymeans for directing the other ofsaid rays through the evacuated chamber, Vreflecting means, saiddividing means, directing ,means and reiiecting means causing said raysto be recombined, metallic means for supporting the members determiningthe path'of the ray passing through the atmosphere filled chamber, andmeans, comprising a metallic portion and a quartz portion, for supquartzportion having a lesser c oemcient of theri,

mal expansion than the metallic means and causing the geometric lengthof the path of the latter ray to 'be increased less than the path of theformer rayby increases in temperature.

.12. In interferometric means for measuring the l difference inpressurebetween the mediums in a chamber containing atmospheric air andan evacuated chamberfa source of light, meansfor dividing a beam oflight from said source into two rays and for directing one of said raysthrough the atmosphere lled chamber, means for directing the other ofsaid rays through the )causingJ said rays to -be recombined, metallicmeans for mounting thev members which determine the path of the raypassing through'v the atmosphere lled chamber, and means for mountingthe members determining the path of the 'ray through the evacuatedchamber, said lastnamed mounting means having a smaller aver` agecoeflicient of thermal expansion than said mst-named mounting meanswhereby the path of .the ray passing through the atmosphere lled chamberis increased in length a greater by increases in temperature.

13. In a measuring instrument utilizing light interference as a basisfor measuring the difierence in pressure between equal lengths of twomediums, means for separately containing said mediums A'to provide equallengths thereof as paths for light rays, said means comprising a memberprovided with parallel end surfaces and having openings thereinextending the length thereof, and a pair of transparent plates havingparallel faces, one of said plates being wrung adapted to varythe'optical length of the light means foi` measuring the pressuredifference be'- tween two mediums.' a chamberv of known length adaptedlto be evacuated to provide 'a known amount length of comparison medium,ymeans for evacuating said chamber, -a Geissler tube communieating withsaid chambenand means for applying a high potential across theterminals'of said tube to visually indicate the degree of vacuumexisting in said chamber.

15. In a measuring instrument utilizing light interference as a vbasisfor measurement, a member of known length having parallel endsurfaces,

said member having a pair of openings therein extending the lengththereof, a pair of transparent plates, one of said plates being Wrung toeach end of said member to form chambers of vsaid openings', meansvconnecting one of said chambers with the atmosphere, and means forproviding the other of said chambers with a medium of known index ofrefraction.

' ROY T. HURLEY.

